Optical isolator and method for assembling same

ABSTRACT

In optical isolators composed of a polarizer, an analyzer, a Faraday rotator and permanent magnet for magnetizing the Faraday rotator for the Faraday effect, this invention enables controlling of the maximum isolation temperature in the 0° to 70° C. temperature range. When assembling the device at room temperature, the Faraday rotator&#39;s wavelength-dependence characteristics are used for this purpose. By varying the wavelength during the assembly and adjustment process by Δλ from the wavelength at the device will be used, it is possible to set the temperature at which maximum isolation will be realized.

FIELD OF TECHNOLOGY

This invention concerns an optical isolator which uses temperaturedependence to control the maximum isolation temperature.

BACKGROUND TECHNOLOGY

With the development of long-distance optic communications technologyusing LD (semiconductor lasers), optical isolators have been developedin order to prevent LD noise from reflected, returning light. With theproliferation of high density communications systems, the importance ofthese devices has grown considerably. The basic structure consists of aFaraday rotator composed of two polarizers and a garnet crystal, alongwith a permanent magnet to produce the Faraday effect by themagnetization of the Faraday rotator. For the polarizer, one could use aRochon polarizing prism, polarized beam splitter, Grant-Thompson+prism,or polarized glass depending upon application. In order to achieve smallsize and a high magnetic field, a rare-earth permanent magnet is used.The Faraday rotator was the part which determined the characteristics ofthe optical isolator. There are currently two types of materials beingused. One is where the FZ method is used to produce a bulk YIG (1/2Fe₅O₁₂) single crystal and the other is where the liquid phase epitaxial(LPE) method is used in order to produce a BiRIG (rare earthbismuth-ison garnet) film on a garnet type of substrate. The Faradayrotation angle θ_(f) is proportional to the thickness of the crystals,and θ_(f) per unit of length is different from each material. In orderto obtain an angle θ_(f) =45° as required for an optical isolator, theYIG should be about 2 mm, and the BiRIG should be 200 to 500 μm. Inconsideration of mass production and lowered costs, after the FZ methodhas been used to obtain the bulk YIG, in order to produce the requiredshape on the substrate using the LPE method without wasteful machining,it is possible to produce a large film 1/4 of the thickness butequivalent to the YIG in the Faraday rotator. This is of greet benefitfor the economic proliferation of these elements. However, in u theseYIG and LPE garnet crystals, there have some differences in opticalcharacteristics which result in temperature or wavelength dependence inthe Faraday rotation. FIG. shows the temperature dependence (a) and thewavelength dependence (b) of θ_(f) for a BiRIG. Depending on thematerials, there are variations in the reverse slope, but generally, theθ_(f) corresponds to temperature and wavelength. Optical isolators areadjusted and assembled to have a maximum isolation at the wavelength andthe ambient temperature which they are assembled. However, intemperature ranges from 0° to 70° C. where these devices will bepractically used, near the temperatures at either end of this range (0°C. and 70° C.) the isolation characteristics tend to deteriorate. In YIGcrystals, the Faraday rotation temperature dependence coefficient isgenerally K_(T) =-0.04 deg/° C. With the LPE method, in materials wherethere is essentially little absorption, it is K_(T) =-0.04 to -0.07deg/° C. When the optical isolators are assembled at room temperature(about 23° C.), they are adjusted so that θ_(f) =45° at 23° C. However,if we assume the temperature coefficient of an LPE garnet element to be-0.07 deg/° C., then at the upper end of 23° C. ±20° C. (eg. 43° C.),θ_(f) =45 -0.07×20=43.6°. At the lower limit of 3° C., this becomes46.4°. This greatly degrades the isolation due to the slippage fromθ_(f) =45°. In principle, isolation is -10 Log[sin² (45-θ_(f))], so inthe previous example, at 3° C. and 43° C. it would be 32 dB. The graphin FIG. 2 (1) shows the temperature dependence of isolation for atypical optical isolator. The peak of isolation is at 23° C., and itfalls off below and above that temperature in a nearly symmetricalcurve. In this case, when considering a temperature range from 0° to 76°C., at the limit temperatures, the Faraday rotation angle is 44 deg for24→0° C. and 46 deg from 24→70° C. Table 1 shows the isolation at 0° C.and 70° C. when the isolation has an angular displacement angle Δθ from45° according to -10 log (sin² Δθ) for YIG and LPE garnets (when thetemperature coefficient=-0.06 deg/°C.).

                  TABLE 1                                                         ______________________________________                                                Temp. Coefficient                                                                           Isolation (dB)                                                  (deg/°C.)                                                                            0° C.                                                                          70° C.                                   ______________________________________                                        YIG garnet                                                                              -0.04           -35.5   -29.9                                       LPE garnet                                                                              -0.06           -32.0   -26.3                                       ______________________________________                                    

If we assume that 30 dB or greater of isolation is required from O→70°C., then problems would appear with either method at the hightemperature end of the range. In order to reduce this problem, thematerials used in the Faraday rotator should have a low temperaturecoefficient, but at the current time, such low absorption materials arenot available. The following methods can be considered as ways ofproviding high isolation at a temperature range of 0° to 70° C.:

1) Assemble the optical isolators at a temperature mid-way in the aboverange, eg. 35° C.

2) Produce isolators with a maximum isolation temperature of 35° C. bymoving the polarizer angle of rotation Δθ from an angle of 45°.

3) Produce isolators with a maximum isolation temperature of 35° C. bydeviating Δθ from the 45° Faraday rotation angle.

If the first of the above methods were used, an assembly system wouldhave to be established where the ambient temperature was higher thanroom temperature. If the method 2) were adopted, during the mechanicaldisplacement by Δθ from 45°, it would be impossible to fix the peak forthe standard isolation, so the resulting products would have somefluctuation in their characteristics. In the case of 3), it wouldinvolve increasing or reducing the thickness of the Faraday rotator, andthe θ±Δθ adjustment would be complex. In any of the above cases, itwould not be practical to implement such production.

Considering costs and production for a multi-purpose optical isolator(for optic communications of subscribers, Cable TV optic communicationsystems), it would be difficult to meet the requirements except by usingthe LPE method garnet. Thus, there is a demand for optical isolatorswhich are produced using the LPE method but which have little absorptionand are stable in the face of temperature variations.

SUMMARY OF THE INVENTION

In this invention, in the assembly of optical isolators comprised ofpolarizers, analyzers using Farady rotators, and permanent magnets tomagnetize the Faraday rotators to produce the Faraday effect, thewavelength-dependence characteristics of the Faraday rotator areutilized by finely adjusting the wavelength Δλ so that the maximumisolation is exhibited at a temperature which is higher than thetemperature at which the device was assembled.

When the temperature coefficient of Faraday rotation angle is K_(T) andthe wavelength coefficient is K.sub.λ, then the variation in the Faradayrotation angle due to a small temperature change ΔT is adjusted bychanging the wavelength Δλ in order to compensate for this according to:

    K.sub.λ Δλ=K.sub.T ΔT            (1)

This is the equivalent of the change in wavelength: Δλ=(K_(T)/K.sub.λ)ΔT. When optical isolators are normally assembled at roomtemperature (T_(R)), they show their highest isolation capacity at thattemperature. As a result, as shown in FIG. 2 (1), the peak value for theisolation is displaced somewhat to the low temperature side of T_(R).However, if the Δλ of equation (1) is brought into consideration, whenoptical isolators are assembled so that the maximum isolation is at awavelength λ±Δλ at room temperature, then the maximum isolation is atT_(R) ±ΔT when operating at wavelength λ. In other words, if the changein Faraday rotation due to temperature is compensated by changing thewavelength at the time of assembly, the maximum isolation temperaturecould be controlled by changing the wavelength Δλ according to equation(1). In cases where this is a module with the LD, then the LD should beproduced with a tolerance of about ±30 nm. For optical isolators whichare assembled to be used at a certain wavelength, this principle can beused as described. With an LD module, the temperature characteristicstend to vary considerably, so the wavelength of the LD can first bedetermined, and then the isolator can be assembled so that the maximumisolation temperature matches according to the adjusted wavelength asdetermined by Equation (1). This makes it possible to provide LD moduleswhich have uniform temperature characteristics. Further, if maximumisolation is desired at a particular wavelength, then Equation (1) canbe used to determine the assembly temperature at which this can berealized in order to get the characteristics desired at the designatedwavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 contains graphs of the temperature dependence (a) and thewavelength dependence (b) of a BiRIG Faraday rotational angle θ_(f).

FIG. 2 is a graph showing the isolation temperature dependence of anoptical isolator of this invention.

FIG. 3 is a graph showing the isolation temperature dependence of anoptical isolator of this invention.

FIG. 4 is a graph showing the isolation temperature dependence of anoptical isolator in which the assembly temperature is varied.

FIGS. 5 and 6 are graphs of data on isolation temperaturecharacteristics of the optical isolator examples of this invention.

FIG. 7 is a schematic depiction of an optical isolator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 7, an optical isolator arrangement is shown which is composed ofan optical isolator 20 which receive light of wavelength λ from a lightsource such as a semiconductor laser (laser diode) LD. Optical isolator20 comprises a polarizer 21 and a Faraday rotator 22 which has a Faradayrotation element that is magnetized by a permanent magnet. Thepolarization of the light incident on the rotator is rotated so thatreflected light can be cut off by polarizer 21 at the input end whiletransmitted light passes through the analyzer 23 at the exit end of therotator 22. To this extent, FIG. 7 represents a conventional opticalisolator and the inventive method and arrangement will now be describedrelative to the following examples.

EXAMPLE 1

A Faraday element was used with a Faraday rotation capability whichwould allow a maximum isolation temperature of 34° C. as indicated inEquation (2). The isolator was assembled using an assembly wavelength of1540 nm, as determined by Equation (3), at a temperature of 23° C. FIG.2, curve (2) shows the temperature dependence of the resulting opticalisolator when measured at a wavelength of 1550 nm. The maximum isolationtemperature was 34° C. Similarly, (3) shows the curve for an isolatorassembled at 1530 nm, and (4) shows the curve for one assembled at 1560nm. ##EQU1## Where θ_(f) is the Faraday Rotation (deg/cm) at wavelengthλ(nm) ##EQU2## Where: θ_(fc) : The Faraday rotation capability (deg/cm)at the central wavelength

θ_(fk) : The Faraday rotation capability (deg/cm) at the adjustedassembly wavelength

T_(r) : Assembly temperature

K_(T) : Temperature coefficient (deg/°C.) for the Faraday rotator

T_(ps) : Maximum isolation temperature

Thus, in this example, where the actual wavelength is λ, then when aλ±Δλ wavelength light source is used to adjust the maximum isolationangle, the desired maximum isolation temperature can be achieved for theoptical isolator.

EXAMPLE 2

The wavelengths selected for assembly were λ=1310 nm, FIG. 3, curve (5),1318 nm, FIG. 3, curve (6), and 1326 nm, FIG. 3, curve (7) and thetemperature used for adjusting the maximum isolation was 23° C. Afterassembly, the isolation temperature was changed and the isolation wasmeasured at a wavelength of 1310 nm to obtain the results shown in FIG.3. As the assembly wavelength increased, the peak position moved to thehigh temperature side, as shown in the Figure. When a λ=1318 assemblywavelength was used, and measurements taken at λ=1310, the isolation wasabout 30 dB at 0° C. and 29.5 dB at 70° C. When compared to devicesassembled at λ=1310 nm, the peak point was shifted to a position at 35 °C. The temperature coefficient of the Faraday rotator used was -0.06deg/° C. In this case in order to move the peak by 12° C., it wasnecessary to use an assembly wavelength which was different from theusage wavelength by about 8 nm.

EXAMPLE 3

The same parts were used as in Example 2, but the assembly took place inconstant temperature baths of 23° C. FIG. 4, curve (8), 35° C. FIG. 4,curve (9) and 45° C., FIG. 4, curve (10) in setting the maximumisolation. Affixing was performed using a fiber optic-guided YAG weldingmethod in order to attach the isolator to a stainless steel holder. Inthis case, both the assembly .wavelength and the measurement wavelengthwere λ=1310. FIG. 4 shows the characteristics of the optical isolatorswhich were assembled at the above-mentioned temperatures. The unit whichwas assembled at 35° C. had an isolation of 29.0 dB at 0° C. and 29.5 dBat 70° C. Thus, it can be seen that about the same level of isolation isachieved in the optical isolators assembled as in Example 2 as isachieved when they are assembled at various temperatures.

EXAMPLE 4

An optical isolator was envisioned using a 1535 nm wavelength and havinga maximum isolation temperature 45° C. Four devices were prepared usingdifferent assembly wavelengths and then temperature characteristics weremeasured. The LPE method was used in order to obtain a Bi-substitutedrare earth iron garnet for the Faraday rotator. It had a temperaturecoefficient of -0.06 deg/° C. and a Faraday rotation wavelengthcoefficient of about -0.068 deg/nm, so the relationship between ΔT andΔλ was as follows:

    -0.068 (deg/nm)×Δλ(nm)

    -0.065 (deg/° C.)×(45-23) (° C.)

Therefore, Δλ=about 22nm. Thus, assembly took place using a wavelength22 nm longer than 1535 nm: 557 nm. FIG. 5 shows the temperaturecharacteristics of the 4 optical isolators. The maximum isolation wasnot at 45° C. but fell between 42 and 44° C. Thus, characteristics whichwere close to those forecast were obtained.

EXAMPLE 5

An optical isolator for use at 1310 nm wavelengths was prepared as inExample 4, but assembled using a wavelength 15 nm longer, 1325 nm. FIG.6 shows the temperature characteristics.

Using the adjustment of the assembly wavelength according to thisinvention it is possible to control the maximum isolation temperature asdesired in order to obtain high isolation characteristics in thetemperature range where the device will be used. By setting the maximumisolation in the middle of this temperature range, it is possible tomake effective utilization of extinction properties. When assemblingwith an LD, by using an assembly wavelength corresponding to the LDwavelength, one can easily manufacture optical isolators which areappropriate to the variations in wavelength inherent in LD production.

We claim:
 1. An optical transmission arrangement comprising a source oflight of wavelength λ and an optical isolator having a polarizer, ananalyzer, a Faraday rotator formed of a material having a range ofoptical properties which decrease symmetrically as a function oftemperature from a temperature at which a maximum value of said opticalproperties occurs and a permanent magnet which magnetizes the Faradayrotator into a Faraday rotation angle of approximately 45°; wherein saidoptical isolator has been assembled into an optical position, in whichthe temperature at which said maximum value of said optical propertiesoccurs is room temperature, using a source of light having an adjustedassembly wavelength which minutely differs from the wavelength λ by avalue Δλ in order to shift the temperature at which said maximum valueof said optical properties occurs from said room temperature to atemperature in the middle of a temperature range in which the opticalisolator is used with the light source of wavelength λ.
 2. Thearrangement according to claim 1, wherein Δλ is approximately 5-30 nm.3. An optical transmission arrangement according to claim 2, wherein theFaraday rotation angle of said Faraday rotator has a temperaturecoefficient of K_(T) and a wavelength coefficient of K.sub.λ, andwherein the adjusted assembly wavelength difference value Δλ withrespect to a temperature variation ΔT from the middle of the temperaturerange in which the arrangement is used with the light source ofwavelength λ is:

    Δ=(K.sub.T /K.sub.λ)ΔT

and as a means of setting the ±ΔT, the optical isolator has beenassembled at room temperature T_(R), using an assembly wavelength ofλ±Δλ with respect to wavelength Δ so that maximum isolation will beachieved at the middle of the temperature range in which the arrangementwill be used with the light source of wavelength Δ.
 4. A method ofassembling an optical isolator; having a polarizer, an analyzer aFaraday rotator formed of a material having a range of opticalproperties which decrease symmetrically as a function of temperaturefrom a temperature at which a maximum value of said optical propertiesoccurs and a permanent magnet which magnetizes the Faraday rotator intoa Faraday rotation angle of approximately 45°, for use with a lightsource of light of wavelength λ comprising the steps of:A) determining atemperature range within which the optical isolator will be used; B)selecting a temperature in the middle of the temperature rangedetermined; C) assembling the optical isolator into an optical position,in which said maximum value of said optical properties occurs at roomtemperature, using a light source of a wavelength which differs fromwavelength λ by an amount ±Δλ which will shift the temperature at whichsaid maximum value of said optical properties occurs from said roomtemperature to the selected temperature when the optical isolator isused with the light source of wavelength λ.
 5. The method according toclaim 4, wherein Δλ is selected in accordance with the equation:

    Δλ=(K.sub.T /K.sub.λ)ΔT

where K_(T) is a temperature coefficient of the Faraday rotation angleof the Faraday rotation element, K.sub.λ is a wavelength coefficient,and ΔT is a temperature variation within the temperature range from theselected temperature.